Technical Field
This application relates to recovering from metadata inconsistencies in storage systems.
Description of Related Art
Computer systems may include different resources used by one or more host processors. Resources and host processors in a computer system may be interconnected by one or more communication connections. These resources may include, for example, data storage devices such as those included in the data storage systems manufactured by EMC Corporation. These data storage systems may be coupled to one or more servers or host processors and provide storage services to each host processor. Multiple data storage systems from one or more different vendors may be connected and may provide common data storage for one or more host processors in a computer system.
A host processor may perform a variety of data processing tasks and operations using the data storage system. For example, a host processor may perform basic system I/O operations in connection with data requests, such as data read and write operations.
Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units, disk drives, and disk interface units. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels to the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units. The logical disk units may or may not correspond to the actual disk drives. Allowing multiple host systems to access the single storage device unit allows the host systems to share data in the device. In order to facilitate sharing of the data on the device, additional software on the data storage systems may also be used.
Such a data storage system typically includes processing circuitry and a set of disk drives (disk drives are also referred to herein as simply “disks” or “drives”). In general, the processing circuitry performs load and store operations on the set of disk drives on behalf of the host devices. In certain data storage systems, the disk drives of the data storage system are distributed among one or more separate disk drive enclosures (disk drive enclosures are also referred to herein as “disk arrays” or “storage arrays”) and processing circuitry serves as a front-end to the disk drive enclosures. The processing circuitry presents the disk drive enclosures to the host device as a single, logical storage location and allows the host device to access the disk drives such that the individual disk drives and disk drive enclosures are transparent to the host device.
Disk arrays are typically used to provide storage space for one or more computer file systems, databases, applications, and the like. For this and other reasons, it is common for disk arrays to be structured into logical partitions of storage space, called logical units (also referred to herein as LUs or LUNs). For example, at LUN creation time, storage system may allocate storage space of various storage devices in a disk array to be presented as a logical volume for use by an external host device. This allows a disk array to appear as a collection of separate file systems, network drives, and/or volumes.
Disk arrays may also include groups of physical disks that are logically bound together to represent contiguous data storage space for applications. For example, disk arrays may be divided into redundant array of inexpensive disks (RAID) groups, which are disk arrays created by logically binding individual physical disks together to form the RAID groups. RAID groups represent a logically contiguous address space distributed across a set of physical disks. Each physical disk is subdivided into pieces used to spread the address space of the RAID group across the group (along with parity information if applicable to the RAID level). The physically contiguous pieces of the physical disks that are joined together to create the logically contiguous address space of the RAID group are called stripes. Stripes may form blocks and blocks may be allocated to create logical representations of storage space for use by applications within a data storage system.
As described above, applications access and store data incrementally by use of logical storage array partitions, known as logical units (LUNs). LUNs are made up of collections of storage blocks of a RAID array and are exported from the RAID array for use at the application level.
Existing data storage systems may utilize different techniques in connection with managing data availability in data storage systems, for example, in the event of a data storage device failure. There are a number of different RAID (Redundant Array of Independent or Inexpensive Disks) levels and techniques that may be used in connection with providing a combination of fault tolerance and/or improved performance for data storage devices. Different RAID levels (e.g., RAID-1, RAID-5, RAID-6, and the like) may provide varying degrees of fault tolerance. Further, RAID parity schemes may be utilized to provide error detection during the transfer and retrieval of data across a storage system.
Large storage arrays today manage many disks that are not identical. Storage arrays use different types of disks, i.e., disks with different RAID (Redundant Array of Independent or Inexpensive Disks) levels, performance and cost characteristics.
Generally, a RAID system is an array of multiple disk drives which appears as a single drive to a data storage system. A goal of a RAID system is to spread, or stripe, a piece of data uniformly across disks (typically in units called chunks), so that a large request can be served by multiple disks in parallel. For example, RAID-5 techniques can be used in connection with a data storage system to protect from a single device failure.
In a particular RAID-5 context, for example, which comprises a storage array of five disk modules, each disk has a plurality of “N” data storage sectors, corresponding sectors in each of the five disks being usually referred to as a “stripe” of sectors. With respect to any stripe, 80% of the sector regions in the stripe (i.e., in a 5 disk array effectively 4 out of 5 sectors) is used for user data and 20% thereof (i.e., effectively 1 out of 5 sectors) is used for redundant, or parity, data. The use of such redundancy allows for the reconstruction of user data in the event of a failure of a user data sector in the stripe.
When a user data disk module fails, the redundant or parity entry that is available in the parity sector of a stripe and the data in the non-failed user data sectors of the stripe can be used to permit the user data that was in the sector of the failed disk to be effectively reconstructed so that the system can remain operative using such reconstructed data even when the user data of that sector of the failed disk cannot be accessed. The system is then said to be operating in a “degraded” mode since extra processing operations and, accordingly, extra time is required to reconstruct the data in the failed disk sector when access thereto is required.
Certain kinds of failures, however, can occur in which the storage array is left in an incoherent or effectively unusable state, e.g., a situation can occur in which there is power failure, i.e., power to a storage processor fails or the storage processor itself fails due to a hardware or software defect, or power to the disk drives themselves fails.
In data storage systems where high-availability is a necessity, system administrators are constantly faced with the challenges of preserving data integrity and ensuring availability of critical system components. One critical system component in any computer processing system is its file system. File systems include software programs and data structures that define the use of underlying data storage devices. File systems are responsible for organizing disk storage into files and directories and keeping track of which part of disk storage belong to which file and which are not being used.
The accuracy and consistency of a file system is necessary to relate applications and data used by those applications. However, there may exist the potential for data corruption in any computer system and therefore measures are taken to periodically ensure that the file system is consistent and accurate. In a data storage system, hundreds of files may be created, modified, and deleted on a regular basis. Each time a file is modified, the data storage system performs a series of file system updates. These updates, when written to disk storage reliably, yield a consistent file system. However, a file system can develop inconsistencies in several ways. Problems may result from an unclean shutdown, if a system is shut down improperly, or when a mounted file system is taken offline improperly. Inconsistencies can also result from defective hardware or hardware failures. Additionally, inconsistencies can also result from software errors or user errors.
Additionally, the need for high performance, high capacity information technology systems are driven by several factors. In many industries, critical information technology applications require outstanding levels of service. At the same time, the world is experiencing an information explosion as more and more users demand timely access to a huge and steadily growing mass of data including high quality multimedia content. The users also demand that information technology solutions protect data and perform under harsh conditions with minimal data loss and minimum data unavailability. Computing systems of all types are not only accommodating more data but are also becoming more and more interconnected, raising the amounts of data exchanged at a geometric rate.
To address this demand, modern data storage systems (“storage systems”) are put to a variety of commercial uses. For example, they are coupled with host systems to store data for purposes of product development, and large storage systems are used by financial institutions to store critical data in large databases. For many uses to which such storage systems are put, it is highly important that they be highly reliable and highly efficient so that critical data is not lost or unavailable.